1. Field of the Invention
The present invention relates to print wheels such as, for example, character wheels used in hot debossing stamper printing machines. More particularly, the invention relates to reliable detection of print wheel fault conditions such as jams and positioning errors. The invention also relates to homing the print wheels and identifying the print wheel type.
2. Description of Related Art
In hot debossing stamper printing machines, type writers and other devices using print wheels, the print wheel is rotated during printing to align the print wheel petal carrying a character with the printer strike hammer. In most applications the print wheel is positioned with an open loop (no feedback) stepper motor. In such a system, there is minimum position feedback associated with the print wheel motion. If the print wheel becomes mispositioned, the print wheel may become damaged and the printing tape may get wasted. For example, if the print wheel gets jammed because, for instance, a petal gets stuck in the print wheel casing or the hammer gets stuck to the petal, severe damage may result when the motor tries to rotate the print wheel. Further, if the print wheel becomes mispositioned because, for example, of a momentary mechanical interference with the casing or the strike hammer, wrong characters will be printed resulting in tape waste. Thus, an early and reliable error detection allowing aborting the printing early when a fault occurs, especially if printing is unattended, is highly desirable.
In one error detection technique, a reflective strip is placed at the base of a "home" petal and other reflective and non-reflective strips are placed near the home petal in a circular arc of, for example, 80.degree. to 90.degree.. The other non-reflective strips identify the print wheel type--the same printing engine may be able to accept different print wheel types with, for example, different fonts. In such a system, the home flag (i.e., the home petal strip) might be readable by an optical sensor on phase A of the print wheel stepper motor and the print wheel type strips may be readable on phase C. When the print wheel is first inserted into the printing engine, the engine rotates the print wheel, determines the home position by reading the sensor on phase A of the stepper motor, and then reads the print wheel type on phase C. The engine then "homes" the print wheel by positioning the home strip beneath the optical sensor. Later, during printing, whenever the print wheel is expected to be in its home position, the engine reads the sensor to make sure that the sensor sees the reflective strip. If the sensor does not see a reflective strip, an error is reported to the operator. The printing stops, allowing the operator to take a corrective action. One type of an encoded print wheel is described in U.S. Pat. No. 4,074,798 issued Feb. 21, 1978 to M. Berger.
In a second error detection technique, only the home reflective strip is present and the above described type information is not utilized.
A drawback of these error detection schemes is that the print wheel could become jammed or mispositioned before it was supposed to be in the home position. The printer would continue to print, potentially ruining the print wheel. Moreover, if there are other reflective strips besides the home strip, print wheel mispositioning could escape detection even when the print wheel was supposed to be in its home position because the sensor could mistake another reflective strip for the home strip.
Thus, there is a need for a more reliable print wheel error detection than the error detection provided by prior techniques.